interact, which points to the corresponding unix_interact.
(do_kill_window_subscriber): New function, which updates the
number of subscribers and removes the SIGWINCH handler if there
are no subscribers left.
(unix_interact): Added attributes backend, winch_handler and
nsubscribers.
(unix_window_change_subscribe): Keep track of number of
subscribers, and install the signal handler if it is needed.
(make_unix_interact): Don't install any signal handler yet.

Rev: src/unix_interact.c:1.11